Reduction of particulate matter and nitrogen oxides exhausted from a diesel engine for vehicle such as a bus and truck has drawn a large attention, but elimination of harmful substances in exhaust gas from a diesel engine for ships and power generation, boiler exhaust gas, and plant off-gas is similarly an important problem. However, while the diesel engine for vehicle uses light oil with smaller sulfur contents as fuel, the diesel engine for ships and power generation and the like uses fuel with high sulfur contents such as Bunker A or Bunker C and a large amount of sulfur oxides is contained in the exhaust gas of the diesel engine for ships and power generation and the like, which makes major impediment in removal treatment of harmful substances.
In general, as a denitration method of exhaust gas, a non-catalyst denitration method and a selective catalytic reduction method (SCR method) are known. As the non-catalyst denitration method, a denitration method by a nitrogen reducing agent such as ammonia, urea or the like is widely known, but high activity can not be obtained unless an exhaust gas temperature is as high as 900 to 1000° C. (See Patent Documents 1 and 2, for example). Thus, pretreatment such as heating of the exhaust gas to raise the temperature is needed for the exhaust gas at a relatively low temperature of approximately 250 to 450° C. exhausted from the diesel engine for ships and power generation and the like, which incurs increase in treatment costs and makes application of the non-catalyst denitration method difficult.
With the non-catalyst denitration method using ammonia, a high denitration rate can be obtained on a lab level, but the denitration rate exceeding 50% has been difficult for combustion furnaces such as an actual boiler. For example, non-patent document 1 discloses a denitration method that an ammonia supply amount is divided into pre and post two stages according to a boiler load in a temperature range of 900 to 1050° C. and each of them is supplied in a controlled manner. However, its denitration rate is only approximately 40% under an equimolar condition of nitrogen oxides and ammonia. Moreover, if ammonia is excessively supplied with a purpose of increasing the denitration rate, unreacted ammonia is left, by which the treatment cost is raised. And if the exhaust gas contains sulfur oxides, it generates ammonium sulfate, which requires treatment and deteriorates cost effectiveness.
On the other hand, Patent Document 3 proposes such a method as the SCR method that nitrogen monoxide in the exhaust gas is oxidized into nitrogen dioxide and then, a reducing agent such as ammonia, urea, hydrocarbon or the like is added in presence of SCR catalyst for catalytic reduction. However, the SCR method is poorer in performance than the non-catalyst denitration method in a point that a large amount of SCR catalyst is used and moreover, if the exhaust gas temperature is 300° C. or below, sulfur trioxide or the like generated from oxidation of sulfur dioxide in the exhaust gas reacts with ammonia and generates ammonium hydrogen sulfate to poison the SCR catalyst, by which catalytic activity is lowered. Thus, the SCR method has been used only when the exhaust gas is in a high temperature state of 300° C. or above at which ammonium hydrogen sulfate is decomposed or a sulfur oxidation concentration in the exhaust gas is approximately 1 ppm or less.
Among them, Patent Document 4 proposes a method in which a heating region is formed in a flue through which a low-temperature exhaust gas containing sulfur oxides passes or in a chamber communicating with the flue, amine radicals are generated by blowing nitrogen compounds and hydrocarbons into the heating region, and the nitrogen oxides in the exhaust gas is denitrated by the amine radicals. However, the denitration rate by this denitration method is not necessarily sufficient, and improvement of the denitration rate has been in demand.
Patent Document 1: U.S. Pat. No. 6,066,303
Patent Document 2: Japanese patent application Kokai publication No. 2002-136837
Patent Document 3: Japanese patent application Tokizhyou publication No. 2001-525902
Patent Document 4: Japanese patent application Kokai publication No. 2005-254093
Non-patent Document 1: “Fuel conversion and technology for SOX/NOX measures—mainly on smoke exhaust desulphurization/denitration” by JunpeiAndo, Project News Company, Jun. 25, 1983, pp. 205 to 207